In recent years, input apparatuses called touch panels have been widely used for automatic teller machines in financial institutions, ticketing machines in railroad stations, information processing apparatuses such as PDAs (Personal Digital Assistants), and so forth. The touch panel type input apparatus correlates icons and so forth displayed on a display such as an LCD (Liquid Crystal Display) with the coordinate system on the display panel and displays the position that the user has touched with his or her finger or a pointing device such as a pen so as to accomplish a GUI (Graphical User Interface) function.
In the conventional touch panel type input apparatus, when the user operates it and inputs data thereto, for example an icon that he or she has pressed varies in its appearance or an operation sound occurs so as to inform him or her that the input operation has been accepted. In contrast, in a recent touch panel, when the user presses an icon or the like, the height of the panel varies, causing a force sense to be fed back to his or her finger or a pointing device. As a result, since the user can feel as if he or she touched (clicked) a switch button, his or her operational feeling will improve.
For example, Japanese Patent Application Laid-Open Publication No. 2002-259059 (laid open on Sep. 13, 2002) discloses a resistor film type touch panel having a structure of which a plurality of electrode sheets on which transparent electrodes are formed are layered in such a manner that they are spaced apart by a predetermined length and their electrodes are oppositely aligned. In particular, the specification and FIG. 6 of this related art describe a touch panel having a multilayer structure, three or more electrode sheets, and actuators such as bobbin coils disposed between a casing that fixes the touch panel and a casing that fixes the display side so as to cause the user to feel a force sense as a feedback.
In addition, Japanese Patent Application Laid-Open Publication No. HEI 11-212725 (laid open on Aug. 6, 1999) discloses an information display apparatus that uses a piezoelectric device to cause the user to feel a force sense as a feedback and detect an operation input position. In particular, the specification and FIG. 3 of this related art describe an operation panel that is disposed on a liquid crystal display panel and that is supported by a piezoelectric device. The operation force and operation position are detected corresponding to a voltage that is generated in the piezoelectric device when the operation surface of the operation panel is pressed. When operation force larger than a predetermined threshold value is detected, a radio frequency wave is applied to the piezoelectric device to vibrate the operation surface.
The easiest method that allows the user to maximally feel a click sense with the force sense feedback function is to maximize the deformation amount of the touch panel. When the touch panel is deformed with the piezoelectric actuator in such a manner that the piezoelectric actuator is maximally curved in one direction and then maximally curved in the reverse direction, the maximum deformation amount of the touch panel is obtained. However, when the piezoelectric actuator is used in this manner, it is always necessary to maximally curve the piezoelectric actuator in one of two directions in the standby state. Since the curvature amount of the piezoelectric actuator is proportional to the voltage applied thereto, the power consumption of the piezoelectric actuator becomes large. In addition, the life of the piezoelectric actuator becomes short.